The molecular mechanism for human metallothionein-3 to protect against the neuronal cytotoxicity of Aβ1–42 with Cu ions

Serum albumin acted as an effective carrier to improve the stability of bioactive flavonoid

The health-improving functions of bioactive flavonoids in vitro and in vivo are often limited by their low stability, which could be counteracted by the application of proteins as carriers of flavonoids. Clarification of the mechanism of protein-ligand interaction is crucial for the encapsulation of bioactive components. Herein, common plasma proteins [i.e., bovine serum albumin (BSA), human serum albumin (HSA), human immunoglobulin G (IgG) and fibrinogen (FG)] were compared for their binding characteristics to quercetin, the main component of flavonoids in human diet, in the absence and presence of free Cu2+ (an accelerator for flavonoids' instability) using multi-spectroscopic and computational methods. As a flexible open structure of proteins, both BSA and HSA were found to be the most promising carriers for quercetin and Cu2+ with an affinity on the order of 104 M-1. HSA-diligand complex (i.e., HSA-quercetin-Cu2+) was successfully generated when both quercetin and Cu2+ were added to the HSA solution. The stability and free radical scavenging activity of bioactive quercetin during incubation was promoted in the HSA-diligand complex relative to quercetin-Cu2+ complex. Quercetin/Cu2+ system could induce the formation of reactive oxygen species such as hydrogen peroxide (H2O2) and hydroxide radical (·OH), which were significantly suppressed upon HSA binding. Consistently, the cytotoxicity of the quercetin/Cu2+ system to endothelial cells was reduced in the HSA-diligand complex. These results demonstrate the possibility of developing serum albumin-based carriers for the protection of bioactive flavonoids in their nutritional application.

Metal binding and interdomain thermodynamics of mammalian metallothionein-3: enthalpically favoured Cu+ supplants entropically favoured Zn2+ to form Cu4 + clusters under physiological conditions

Metallothioneins (MTs) are a ubiquitous class of small metal-binding proteins involved in metal homeostasis and detoxification. While known for their high affinity for d¹⁰ metal ions, there is a surprising dearth of thermodynamic data on metals binding to MTs. In this study, Zn²⁺ and Cu⁺ binding to mammalian metallothionein-3 (MT-3) were quantified at pH 7.4 by isothermal titration calorimetry (ITC). Zn²⁺ binding was measured by chelation titrations of Zn₇MT-3, while Cu⁺ binding was measured by Zn²⁺ displacement from Zn₇MT-3 with competition from glutathione (GSH). Titrations in multiple buffers enabled a detailed analysis that yielded condition-independent values for the association constant (K) and the change in enthalpy (ΔH) and entropy (ΔS) for these metal ions binding to MT-3. Zn²⁺ was also chelated from the individual α and β domains of MT-3 to quantify the thermodynamics of inter-domain interactions in metal binding. Comparative titrations of Zn₇MT-2 with Cu⁺ revealed that both MT isoforms have similar Cu⁺ affinities and binding thermodynamics, indicating that ΔH and ΔS are determined primarily by the conserved Cys residues. Inductively coupled plasma mass spectrometry (ICP-MS) analysis and low temperature luminescence measurements of Cu-replete samples showed that both proteins form two Cu₄ ⁺-thiolate clusters when Cu⁺ displaces Zn²⁺ under physiological conditions. Comparison of the Zn²⁺ and Cu⁺ binding thermodynamics reveal that enthalpically-favoured Cu⁺, which forms Cu₄ ⁺-thiolate clusters, displaces the entropically-favoured Zn²⁺. These results provide a detailed thermodynamic analysis of d¹⁰ metal binding to these thiolate-rich proteins and quantitative support for, as well as molecular insight into, the role that MT-3 plays in the neuronal chemistry of copper.

Bovine Serum Albumin as a Potential Carrier for the Protection of Bioactive Quercetin and Inhibition of Cu(II) Toxicity

Considering the protective ability of proteins and the potential toxicity of free Cu(II), it was proposed herein that the co-presence of protein could play an important role in suppressing the toxicity of free Cu(II) to the stability of bioactive quercetin if a flavonoid-protein-Cu(II) complex could be formed. In this study, the interaction between quercetin (a major flavonoid in the human diet) and bovine serum albumin (BSA) was investigated in the absence and presence of free Cu(II). The results demonstrated that both quercetin and free Cu(II) had a strong ability to quench the intrinsic fluorescence of BSA through a static procedure (i.e., formation of a BSA-monoligand complex). Site marker competitive experiments illustrated that the binding of both quercetin and Cu(II) to BSA mainly took place in subdomain IIA. The quenching process of free Cu(II) with BSA was easily affected by quercetin, and the increased binding capacity possibly resulted from the generation of a ternary quercetin-BSA-Cu(II) complex. The stability and free radical scavenging activity of bioactive quercetin during incubation was promoted in the BSA-diligand complex relative to a quercetin-Cu(II) complex. A quercetin-Cu(II) system could generate reactive oxygen species such as hydrogen peroxide (H₂O₂) and hydroxyl radicals (^•OH), which were significantly inhibited upon BSA binding. Consistently, the cytotoxicity of the quercetin-Cu(II) system to endothelial cells was decreased in the BSA-diligand complex, where the co-presence of BSA played an important role. These results suggest the possibility and advantage of developing albumin-based carriers for the protection of bioactive components and suppression of Cu(II) toxicity in their biomedical and nutritional applications.

His-Rich Domain of Selenoprotein P Ameliorates Neuropathology and Cognitive Deficits by Regulating TrkB Pathway and Zinc Homeostasis in an Alzheimer Model of Mice

Selenoproteins are a family of special proteins that contain the 21st amino acid, selenocysteine (Sec), in their sequence. Selenoprotein P has 10 Sec residues and modulates selenium homeostasis and redox balance in the brain. Previously, we found that the Sec-devoid His-rich motif of selenoprotein P (Selenop-H) suppressed metal-induced aggregation and neurotoxicities of both Aβ and tau in vitro. To investigate the intervening capacity of Selenop-H on the neuropathology and cognitive deficits of triple transgenic AD (3 × Tg-AD) mice, the Selenop-H gene packaged in rAAV9 was delivered into the hippocampal CA3 regions of mice via stereotaxic injection. Four months later, we demonstrated that Selenop-H (1) improved the spatial learning and memory deficits, (2) alleviated neuron damage and synaptic protein loss, (3) inhibited both tau pathology and amyloid beta protein (Aβ) aggregation, (4) activated both BDNF- and Src-mediated TrkB signaling, and (5) increased MT3 and ZnT3 levels and restored Zn²⁺ homeostasis in the mice model of AD. The study revealed that Selenop-H is potent in ameliorating AD-related neuropathology and cognitive deficits by modulating TrkB signaling and Zn²⁺ homeostasis.

Functional dissection of astrocyte-secreted proteins: Implications in brain health and diseases

Astrocytes, which are homeostatic cells of the central nervous system (CNS), display remarkable heterogeneity in their morphology and function. Besides their physical and metabolic support to neurons, astrocytes modulate the blood-brain barrier, regulate CNS synaptogenesis, guide axon pathfinding, maintain brain homeostasis, affect neuronal development and plasticity, and contribute to diverse neuropathologies via secreted proteins. The identification of astrocytic proteome and secretome profiles has provided new insights into the maintenance of neuronal health and survival, the pathogenesis of brain injury, and neurodegeneration. Recent advances in proteomics research have provided an excellent catalog of astrocyte-secreted proteins. This review categorizes astrocyte-secreted proteins and discusses evidence that astrocytes play a crucial role in neuronal activity and brain function. An in-depth understanding of astrocyte-secreted proteins and their pathways is pivotal for the development of novel strategies for restoring brain homeostasis, limiting brain injury/inflammation, counteracting neurodegeneration, and obtaining functional recovery.

The Efficacy and Pharmacological Mechanism of Zn7MT3 to Protect against Alzheimer's Disease

Alzheimer’s disease (AD) is one of the leading causes of death for people over 65 years. Worse still, no completely effective therapeutic agent is available so far. One important pathological hallmark of AD is accumulated amyloid-β (Aβ) plaques with dysregulated metal homeostasis. Human metallothionin 3 (MT3), a regulator of metal homeostasis, is downregulated at least 30% in AD brain. So far, some in vitro studies demonstrated its multiple functions related to AD. However, it is a great pity that systematic in vivo studies of MT3 on AD model animals are still a blank so far. In this study, we treated APP/PS1 mice with sustained drug release of Zn₇MT3 directly to the central nervous system, and investigated the role and molecular mechanism of Zn₇MT3 to protect against AD mice systematically. The results demonstrated that Zn₇MT3 can significantly ameliorate cognitive deficits, regulate metal homeostasis, abolish Aβ plaque load, and reduce oxidative stress. Additionally, it has been confirmed that MT3 is penetrable to the blood brain barrier of AD mice. All these results support that Zn₇MT3 is an effective AD suppressing agent and has potential for applications in Alzheimer’s disease therapy.

Residue Modification and Mass Spectrometry for the Investigation of Structural and Metalation Properties of Metallothionein and Cysteine-Rich Proteins

Structural information regarding metallothioneins (MTs) has been hard to come by due to its highly dynamic nature in the absence of metal-thiolate cluster formation and crystallization difficulties. Thus, typical spectroscopic methods for structural determination are limited in their usefulness when applied to MTs. Mass spectrometric methods have revolutionized our understanding of protein dynamics, structure, and folding. Recently, advances have been made in residue modification mass spectrometry in order to probe the hard-to-characterize structure of apo- and partially metalated MTs. By using different cysteine specific alkylation reagents, time dependent electrospray ionization mass spectrometry (ESI-MS), and step-wise “snapshot” ESI-MS, we are beginning to understand the dynamics of the conformers of apo-MT and related species. In this review we highlight recent papers that use these and similar techniques for structure elucidation and attempt to explain in a concise manner the data interpretations of these complex methods. We expect increasing resolution in our picture of the structural conformations of metal-free MTs as these techniques are more widely adopted and combined with other promising tools for structural elucidation.

Expression of metallothionein 3 in ductal breast cancer

Metallothionein 3 (MT-3) has the ability to regulate the growth of nerve cells, but the significance of MT-3 expression outside the central nervous system and its participation in carcinogenesis have not yet been clarified. The aim of our study was to investigate the expression of MT-3 in ductal breast cancer and to determine its relationship with well-defined clinicopathological factors in this type of tumor. The study was conducted on 134 cases of invasive ductal breast carcinoma (IDC), 42 samples of non-malignant breast tissue (NMBT), and 26 cases of mastopathy. Moreover, selected breast cancer cell lines (MCF-7, SKBR-3, MDA-MB-231, BO2) and normal human breast epithelial cells (hTERT-HME1) were used. The expression of MT-3 was examined on the protein level using immunohistochemistry and on the mRNA level using real-time PCR. It was shown that the MT-3 protein in cells of IDC and mastopathy appeared in the cytoplasm as well as in the cell nuclei. Both the cytoplasmic and nuclear expression of MT-3 was significantly lower in IDC than in the mastopathies (p<0.0001 and p<0.001). However, no significant correlation was demonstrated between the level of MT-3 protein and the studied clinicopathological factors. The mRNA expression of MT-3 in IDC was also lower than in non‑malignant breast tissue (p<0.0001). Furthermore, in the cases of IDC with lymph node metastasis, the level of MT-3 mRNA was significantly lower than in the cases without metastasis (p=0.0199). The expression of MT-3 mRNA in breast cancer cell lines was significantly lower than in the normal human breast epithelial cell line (p<0.001). These results suggest that MT-3 may play a role in the malignant transformation of breast epithelial cells and in tumor progression.

Metals and Neuronal Metal Binding Proteins Implicated in Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent age-related dementia affecting millions of people worldwide. Its main pathological hallmark feature is the formation of insoluble protein deposits of amyloid-β and hyperphosphorylated tau protein into extracellular plaques and intracellular neurofibrillary tangles, respectively. Many of the mechanistic details of this process remain unknown, but a well-established consequence of protein aggregation is synapse dysfunction and neuronal loss in the AD brain. Different pathways including mitochondrial dysfunction, oxidative stress, inflammation, and metal metabolism have been suggested to be implicated in this process. In particular, a body of evidence suggests that neuronal metal ions such as copper, zinc, and iron play important roles in brain function in health and disease states and altered homeostasis and distribution as a common feature across different neurodegenerative diseases and aging. In this focused review, we overview neuronal proteins that are involved in AD and whose metal binding properties may underlie important biochemical and regulatory processes occurring in the brain during the AD pathophysiological process.

Genetic variation in metallothionein and metal-regulatory transcription factor 1 in relation to urinary cadmium, copper, and zinc

BACKGROUND

Metallothionein (MT) proteins play critical roles in the physiological handling of both essential (Cu and Zn) and toxic (Cd) metals. MT expression is regulated by metal-regulatory transcription factor 1 (MTF1). Hence, genetic variation in the MT gene family and MTF1 might influence excretion of these metals.

METHODS

321 women were recruited in Seattle, WA and Las Cruces, NM and provided demographic information, urine samples for measurement of metal concentrations by mass spectrometry and creatinine, and blood or saliva for extraction of DNA. Forty-one single nucleotide polymorphisms (SNPs) within the MTF1 gene region and the region of chromosome 16 encoding the MT gene family were selected for genotyping in addition to an ancestry informative marker panel. Linear regression was used to estimate the association of SNPs with urinary Cd, Cu, and Zn, adjusted for age, urinary creatinine, smoking history, study site, and ancestry.

RESULTS

Minor alleles of rs28366003 and rs10636 near the MT2A gene were associated with lower urinary Cd, Cu, and Zn. Minor alleles of rs8044719 and rs1599823, near MT1A and MT1B, were associated with lower urinary Cd and Zn, respectively. Minor alleles of rs4653329 in MTF1 were associated with lower urinary Cd.

CONCLUSIONS

These results suggest that genetic variation in the MT gene region and MTF1 influences urinary Cd, Cu, and Zn excretion.

Inhibitory effect of human serum albumin on Cu-induced Aβ(40) aggregation and toxicity

It has been suggested that the aggregation and cytotoxicity of amyloid-β (Aβ) peptide with transition-metal ions in neuronal cells is involved in the development and progression of Alzheimer's disease (AD). As the most abundant protein in blood plasma and in cerebrospinal fluid, human serum albumin (HSA) can bind Aβ in vivo and subsequently inhibit Aβ fibril growth. However, the roles of albumin in Cu-induced Aβ aggregation and toxicity, and its potential biological relevance to AD therapy, were not stressed enough. Here, we showed that HSA was capable of binding Cu (I) with much higher affinity than Aβ, competitively inhibiting the interaction of Aβ and Cu ions. In the presence of biological reducing agent ascorbate, HSA inhibited Cu (II)/Cu (I)-mediated Aβ40 aggregation, reactive oxygen species production, and neurotoxicity. However, in the absence of Cu (II)/Cu (I), HSA could not effectively inhibit Aβ40 aggregation and neurotoxicity at 24 h (or less) incubation time, but decreased Aβ40 aggregation at much longer incubation (120 h). Our data suggested that through competitively decreasing Cu-Aβ interaction, HSA could effectively inhibit Cu (II)/Cu (I)-induced Aβ40 aggregation and neurotoxicity, and play important roles in regulating redox balance as well as metal homeostasis in AD prevention and therapy.

Time dynamics of protein complexes in the AD11 transgenic mouse model for Alzheimer's disease like pathology

Background

Many approaches exist to integrate protein-protein interaction data with other sources of information, most notably with gene co-expression data, to obtain information on network dynamics. It is of interest to look at groups of interacting gene products that form a protein complex. We were interested in applying new tools to the characterization of pathogenesis and dynamic events of an Alzheimer’s-like neurodegenerative model, the AD11 mice, expressing an anti-NGF monoclonal antibody. The goal was to quantify the impact of neurodegeneration on protein complexes, by measuring the correlation between gene expression data by different metrics.

Results

Data were extracted from the gene expression profile of AD11 brain, obtained by Agilent microarray, at 1, 3, 6, 15 months of age. For genes coding proteins in complexes, the correlation matrix of pairwise expression was computed. The dynamics between correlation matrices at different time points was evaluated: paired T-test between average correlation levels and a normalized Euclidean distance with z-score. We unveiled a differential wiring of interactions in a set of complexes, whose network structure discriminates between transgenic and control mice. Furthermore, we analyzed the dynamics of gene expression values, by looking at changes in gene-to-gene correlation over time and identified those complexes that exhibit a different timedependent behaviour between transgenic and controls. The most significant changes in correlation dynamics are concentrated in the early stage of disease, with higher correlation in AD11 mice compared to controls. Many complexes go through dynamic changes over time, showing the role of the dysfunctional immunoproteasome, as early neurodegenerative disease event. Furthermore, this analysis shows key events in the neurodegeneration process of the AD11 model, by identifying significant differences in co-expression values of other complexes, such as parvulin complex, with a role in protein misfolding and proteostasis, and of complexes involved in transcriptional mechanisms.

Conclusions

We have proposed a novel approach to analyze the network structure of protein complexes, by two different measures to evaluate the dynamics of gene-gene correlation matrices from gene expression profiles. The methodology was able to investigate the re-organization of interactions within protein complexes in the AD11 model of neurodegeneration.

Evaluation of eight plasma proteins as candidate blood-based biomarkers for malignant gliomas

Eight brain-derived proteins were evaluated regarding their potential for further development as a blood-based biomarker for malignant gliomas. Plasma levels for glial fibrillary acidic protein, neurogranin, brain-derived neurotrophic factor, intracellular adhesion molecule 5, metallothionein-3, beta-synuclein, S100 and neuron specific enolase were tested in plasma of 23 patients with high-grade gliomas (WHO grade IV), 11 low-grade gliomas (WHO grade II), and 15 healthy subjects. Compared to the healthy controls, none of the proteins appeared to be specific for glioblastomas. However, the data are suggestive of higher protein levels in gliosarcomas (n = 2), which may deserve further exploration.

In vivo-folded metal-metallothionein 3 complexes reveal the Cu-thionein rather than Zn-thionein character of this brain-specific mammalian metallothionein

Metallothionein-3 (MT3) is one of the four mammalian metallothioneins (MT), and is constitutively synthesized in the brain. MT3 acts both intracellularly and extracellularly in this organ, performing functions related to neuronal growth and physiological metal (Zn and Cu) handling. It appears to be involved in the prevention of neurodegenerative disorders caused by insoluble Cu-peptide aggregates, as it triggers a Zn-Cu swap that may counteract the deleterious presence of copper in neural tissues. The literature data on MT3 coordination come from studies either on apo-MT3 reconstitution or the reaction of Zn-MT3 with Cu(2+) , an ion that is hardly present inside cells. To ascertain the MT3 metal-binding features in a scenario closer to the reductive cell cytoplasm, a study of the recombinant Zn(2+) , Cd(2+) and Cu(+) complexes of MT3, βMT3, and αMT3, as well as the in vitro Zn(2+) -Cd(2+) and Zn(2+) -Cu(+) replacement processes, is presented here. We conclude that MT3 has a Cu-thionein character that is stronger than that of the MT1 and MT2 isoforms - also present in the mammalian brain - which is mainly contributed by its β domain. In contrast, the α domain retains a high capacity to bind Zn(2+) ions, and, consequently, the entire MT3 peptide shows a peculiar dual ability to handle both metal ions. The nature of the formed Cu(+) -MT3 complexes oscillates from heterometallic Cu6 Zn4 -MT3 to homometallic Cu10 -MT3 major species, in a narrow Cu concentration range. Therefore, the entire MT3 peptide shows a high capacity to bind Cu(+) , provided that this occurs in a nonoxidative milieux. This reflects a peculiar property of this MT isoform, which accurately senses different Cu contents in the environment in which it is synthesized.

Selenoprotein P and selenoprotein M block Zn2+ -mediated Aβ42 aggregation and toxicity

Aggregation and cytotoxicity of the amyloid-β (Aβ) peptide with transition metal ions in neuronal cells have been suggested to be involved in the progression of Alzheimer's disease (AD). A therapeutic strategy to combat this incurable disease is to design chemical agents to target metal-Aβ species. Selenoproteins are a group of special proteins that contain the 21st amino acid Sec in their sequence. Due to the presence of Sec, studies of this group of proteins are basically focused on their roles in regulating redox potential and scavenging reactive oxygen species. Here, we reported that the His-rich domain of selenoprotein P (SelP-H) and the Sec-to-Cys mutant selenoprotein M (SelM') are capable of binding transition metal ions and modulating the Zn(2+)-mediated Aβ aggregation, ROS production and neurotoxicity. SelM' (U48C) and SelP-H were found to coordinate 0.5 and 2 molar equivalents of Zn(2+)/Cd(2+) with micromolar and submicromolar affinities, respectively. Metal binding induced the structural changes in SelP-H and SelM' according to the circular dichorism spectra. Zn(2+) binding to Aβ42 almost completely suppressed Aβ42 fibrillization, which could be significantly restored by SelP-H and SelM', as observed by thioflavin T (ThT) fluorescence and transmission electron microscopy (TEM). Interestingly, both SelP-H and SelM' inhibited Zn(2+)-Aβ42-induced neurotoxicity and the intracellular ROS production in living cells. These studies suggest that SelP and SelM may play certain roles in regulating redox balance as well as metal homeostasis.